fourth ventricle

n.
  1. An irregular cavity that extends from the obex to a communication with the sylvian aqueduct and is enclosed between the cerebellum dorsally and the tegmentum of rhombencephalon ventrally.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.